We are studying the molecular biology and genetics of avian and murine RNA tumor viruses and of the primate DNA tumor viruses simian virus 40 and Epstein-Barr virus. We are especially interested in the DNA genomes of these viruses and problems related to mechanisms and sites of integration, control of transcription, and mechanisms of transformation by the virus genomes. Dr. Temin is studying the mechanism of formation of the provirus of avian ribodeoxyviruses and testing the protovirus hypothesis. Dr. Mertz is studying the factors controlling transcription of the simian virus 40 genome and the viral functions involved in transforming normal cells to a tumorigenic state. Dr. Risser is studying the genetic transmission of endogenous murine leukemia viruses, modifying genes affecting the production of virus from these endogenous genomes, and the relationship between these endogenous genomes and the susceptibility of mice to chemical carcinogenesis. Dr. Sugden is studying the molecular biology of Epstein-Barr virus. He is mapping the viral genome in cells transformed by different number of virus particles and determining the viral functions responsible for transformation.